My partner is transitioning and I’m happy for them but I miss how they used to look. They’re so much happier but I feel resentful even though I know that’s not fair. I love them so much but I’m scared and I don’t know what to do.
Get out of there. As soon as possible.
When you partnered up it was as they are. If they significantly change whether it's mentally or physically, it's okay to no longer want to be their partner. Both should be understanding and be able to end the relationship without resentment.